Ion mobility spectrometers are used to detect at least selected constituents in a stream of sample gas. An ion mobility spectrometer may be used, for example, to detect the presence of contaminants in air or to detect explosives.
Ion mobility spectrometers have been commercially available since about 1970. An early ion mobility spectrometer is disclosed, for example, in U.S. Pat. No. 3,699,333 which issued to Cohen et al. in 1972. Early work in ion mobility spectrometers also is described by Cohen and Karasek in the Journal of Chromatographic Science, Volume 8 pages 330-337 which was published in 1970. A full review of the theory of ion mobility spectrometry is provided in Mobility and Diffusion of Ions in Gasses by McDaniel and Mason which was published by John Wiley and Sons in 1973. Ion mobility spectrometers produced during the last 20 some years have all been fundamentally the same as the early on mobility spectrometer described by Cohen and Karasek.
A typical prior art ion mobility spectrometer is illustrated schematically in FIG. 1 and is identified generally by the numeral 10. The prior art ion mobility spectrometer 10 is comprised of two parts, namely, the ionization or reaction region 12 and the drift region 14 A sample of air to be analyzed is fed into the ionization region 12 on a stream of air or carrier gas containing a halogenated compound, such as methylene chloride. The carrier or air is ionized by the action of .beta. particles, which typically are emitted from a radioactive nickel.sup.63 source, and which form positive ions and electrons. The electrons are all captured by oxygen or the halogen which is in vast excess in the ionization region 12 of prior art detector 10. The ions which are formed immediately come under the influence of an electric field V.sub.1 in the ionization or reaction region 12, as shown in FIG. 1. The polarity of the field is set to direct the ions of interest (i.e. positive or negative) toward the drift region 14 of the prior art detector 10. For simplicity, only the negative ion analysis will be described here with respect to the prior art detector 10.
Sample molecules that are carried into the ionization or reactor region 12 of the prior art detector 10 may react with the negative ions present if the sample is more electro negative than the negative charge carrier. This type of ion molecule reaction is commonly known as charge transfer. Charge transfer processes can occur in areas of high field strength because there are many opportunities for dissipating the energy in the reacting bodies. However, the charge transfer efficiency in the prior art detector 10 employinq the prior art ion mobility spectrometer technology is very low.
Negative ions, including both sample and reactant ions are attracted toward a shutter grid 16 of the prior art detector 10 as shown in FIG. I. The shutter grid 16 has been essential to all prior art designs of ion mobility spectrometers and was first described by Bradbury and Nielson and was fully explained operationally by McDaniel and Mason in their above referenced work, Mobility and Diffusion of Ions in Gasses. Unfortunately, however, the shutter grid 16 of the prior art detector 10 only allows ions to pass through into the drift and collector region 14 for a short period of time, which typically is about 0.2 mS of duration and which occurs every 20 mS. At all other times, the ions arriving at the shutter grid 16 of the prior art detector 10 shown in FIG. 1 are discharged. This means that of the comparatively few molecules which were ionized, approximately 99% are annihilated in the prior art detector 10 before they can be detected. The total ionization and collection efficiency of the best prior art detector of the type shown in FIG. 1 is less than 0.01%. In view of this inherent inefficiency, prior art ion mobility spectrometers may fail to detect the presence of certain gases of interest which are in fact present in a sample of air being analyzed. For example, prior art ion mobility spectrometers that are used in bomb detectors may be unable to detect many of the organo nitro explosives, such as RDX. Additionally, the reactant ion in the prior art ion mobility spectrometer, usually O.sub.2.sup.- or Cl.sup.-, is likely to mask other light ions, thus making the prior art ion mobility spectrometer unsuitable for detection of such light ions.
In view of these deficiencies of the prior art detectors, it is an object of the subject invention to provide an improved and more efficient ion mobility spectrometer.
It is a further object of the subject invention to provide an ion mobility spectrometer that is much more effective in detecting the presence of explosive ions.
It is a further object of the subject invention to provide an ion mobility spectrometer that is particularly effective in detecting plastic explosives.
Still a further object of the subject invention is to provide a ion mobility spectrometer that is particularly effective for detecting light ions, such as oxygen and oxides of nitrogen and other atmospheric contaminants.